A helicopter is generally provided with at least two turboshaft engines which operate at modes that depend on the flight conditions of the helicopter. Throughout the following text, a helicopter is said to be in a cruise flight situation when it is progressing in normal conditions, in a mode known by the abbreviation AEO (All Engines Operative), during all the flight phases apart from transitional phases of take-off, ascent, landing or hovering flight. Throughout the following text, a helicopter is said to be in a critical flight situation when it is necessary for it to have available the total installed capacity, i.e. during the transitional phases of take-off, ascent, landing and the mode in which one of the turboshaft engines malfunctions, referred to by the abbreviation OEI (One Engine Inoperative).
It is known that, when the helicopter is in a cruise flight situation, the turboshaft engines operate at low power levels, below their maximum continuous thrust (hereinafter MCT). In some arrangements, the power provided by the turboshaft engines during a cruise flight can be less than 50% of the maximum take-off thrust (hereinafter MTO). These low power levels result in a specific consumption (hereinafter SC), which is defined as the relationship between the hourly fuel consumption by the combustion chamber of the turboshaft engine, and the thrust provided by said turboshaft engine, which is approximately 30% greater than the SC of the MTO, and thus an overconsumption of fuel during cruise flight.
Finally, during holding phases on the ground, the pilots generally prefer to put the various turboshaft engines into ground idling so as to be certain of being able to restart them. The turboshaft engines thus continue to consume fuel, despite not providing any power.
Moreover, the turboshaft engines of a helicopter are designed so as to be oversized in order to be able to keep the helicopter in flight in the event of failure of one of the engines. This flight situation corresponds to the OEI mode described above. This flight situation occurs following the loss of an engine, and results in the fact that each functioning motor provides a power that is significantly greater than its rated power in order to allow the helicopter to overcome a dangerous situation, and to then continue its flight. The fuel consumption of each functioning turboshaft engine is therefore significantly increased in the OEI situation in order to provide this power increase.
At the same time, the turboshaft engines are also oversized so as to be able to ensure flight in the entire flight range specified by the aircraft manufacturer, and in particular flight at high altitudes and during hot weather. These flight points, which are very restrictive, in particular when the helicopter has a mass that is close to its maximum take-off mass, are only encountered in specific use cases of specific helicopters. As a result, some turboshaft engines, although dimensioned so as to be able to provide such powers, will never fly in such conditions.
These oversized turboshaft engines have an adverse effect in terms of mass and in terms of fuel consumption. In order to reduce this consumption, in all the cases of flight described above (cruise flight, OEI mode, taxiing, hovering flight, or holding on the ground), it is possible to stop one of the turboshaft engines and to put it into what is known as standby mode. The active engine or engines then operate at higher power levels in order to provide all the necessary power, and therefore at more favourable SC levels. However, this practice is contrary to the current certification rules, and turboshaft engines are not designed to ensure a level of restart reliability that is compatible with safety standards. Likewise, the pilots are not currently aware of or familiar with the idea of putting a turboshaft engine into standby mode during flight.
Furthermore, the restart duration of the turboshaft engine in standby mode is typically approximately thirty seconds. This duration may prove to be too long for some flight conditions, for example at a low flight altitude with a partial or complete malfunction of the initially active engine. If the engine in standby mode does not restart in time, landing using the engine in difficulty may prove to be critical or may even result in a complete loss of power.
More generally, the immediate availability of the power of a single turboshaft engine entails risks in all the flight situations in which it is necessary to provide an increase in power that requires, in terms of safety, to be able to have available the total power of the turboshaft engines.
In FR1151717 and FR1359766, the applicants have proposed methods for optimising the specific consumption of the turboshaft engines of a helicopter by means of the possibility of putting at least one turboshaft engine into a stable flight mode, known as continuous, and at least one turboshaft engine into a particular standby mode that it can leave in an urgent or normal manner, according to need. Leaving the standby mode is said to have occurred normally when a change in the flight situation requires the turboshaft engine in standby to be activated, for example when the helicopter is going to transition from a cruise flight situation to a landing phase. Leaving standby mode normally in this manner occurs over a period of between 10 seconds and 1 minute. Leaving the standby mode is said to have occurred urgently when a failure of or a power deficit in the active engine occurs, or when the flight conditions suddenly become difficult. Leaving standby mode urgently in this manner occurs over a period of less than 10 seconds.
The applicants have thus proposed the following five standby modes:                a standby mode known as normal idling, in which the combustion chamber is ignited and the shaft of the gas generator rotates at a speed of between 60 and 80% of the nominal speed,        a standby mode known as normal super-idling, in which the combustion chamber is ignited and the shaft of the gas generator rotates at a speed of between 20 and 60% of the nominal speed,        a standby mode known as assisted super-idling, in which the combustion chamber is ignited and the shaft of the gas generator rotates, in a mechanically assisted manner, at a speed of between 20 and 60% of the nominal speed,        a standby mode known as banking, in which the combustion chamber is extinguished and the shaft of the gas generator rotates, in a mechanically assisted manner, at a speed of between 5 and 20% of the nominal speed;        a standby mode known as stopping, in which the combustion chamber is extinguished and the engine shaft is completely stopped.        
The technical problem is now that of defining which turboshaft engine needs to be put into standby mode. Another technical problem is that of determining which standby mode should be selected from all of the available standby modes. Another technical problem is that of being able to transition from one standby mode to another, according to the flight conditions of the helicopter. Another technical problem is that of leaving the standby modes and returning to a nominal operating mode.